Voltage-gated sodium channels play a crucial role in maintaining a specific membrane potential (intra- and extracellular ionic environments) across the mammalian cell membrane. The intracellular concentration of Na+ is kept low relative to the extracellular by active sodium pumps that eject three Na+ ions for every two K+ ions taken in. This generates a negative membrane potential (since more positive charge is pumped out and less taken in) and maintains the Na+ concentration of 6 and 140 mM in the intra and extracellular milieu. On opening of the voltage-gated sodium channels (VGSC), Na+ rushes in and leads to depolarization of the membrane because of the associated positive charge. The entry of Na+ via VGSC's occurs in cells of the heart, central and peripheral nervous system and is essential to initiate the firing of an action potential.
VGSCs consist of a pore-forming alpha subunit and a stabilizing beta subunit, 9 isoforms of the alpha subunit have been identified till date (NaV1.1 to NaV1.9). All nine members of the family have >50% identity in the amino acid sequence in the extracellular and transmembrane domain. The channels have also been further classified based on their sensitivity to the puffer fish toxin (tetrodotoxin, TTX). Channels NaV1.8, NaV1.9 and NaV1.5 are TTX resistant (TTX-R) whereas the remaining channels are sensitive to TTX (TTX-S). (England and Rawson. Future Med. Chem. (2010), 2, 775-790). However, NaV1.7 gene is prominently responsible to cause to pain.
Loss of function mutations in the human NaV1.7 gene lead to congenital insensitivity to pain which was observed for the first time in certain Pakistani families. Affected individuals displayed painless burns, fractures, and injuries of the lips and tongue. The patients did not have any autonomic or motor abnormalities, and reportedly had normal tear formation, sweating ability, reflexes, and intelligence. This genetic evidence clearly indicates that gain or loss of NaV1.7 function can lead to exacerbation or loss of pain sensation respectively. Thus, it may be possible to treat chronic pain by pharmacologically blocking NaV1.7. Moreover, NaV1.7 has also been implicated in epilepsy. Small molecule NaV1.7 blockers showed efficacy in in vivo epilepsy models. It has therefore been proposed that selective NaV1.7 blockers may lead to therapeutic benefit in epilepsy (Hoyt et al. Bioorganic & Medicinal Chemistry Letters (2008), 18, 1963-1966).
Genetic evidence stems from the human gain of function as well as loss of function mutations that lead to inherited pain disorders and insensitivity to pain respectively. Non selective VGSC blockers have been shown to alleviate pain in animal models as well as in humans (e.g., Carbamazepine). Ralfinamide, another non-selective sodium channel blocker, is also being developed for the treatment of neuropathic pain.
Voltage-gated sodium channels are implicated in various diseases and disease conditions, including but not limited to chronic pain, visceral pain, arrhythmia, multiple sclerosis, epilepsy and related disorders as well as cancer. Thus, small molecules targeting one or more of the relevant VGSCs is likely to alleviate the suffering from these conditions.
International publication numbers WO 2006/110917, WO 2007/109324, WO 2008/046049, WO 2008/046084, WO 2008/046087, WO 2008/060789, WO 2009/012242, WO 2010/035166, WO 2010/045197, WO 2010/045251, WO 2010/053998, WO 2010/078307, WO2010/151595, WO2010/151597, WO 2011/002708, WO 2011/026240, WO 2011/103196, WO 2011/056985, WO 2011/058766, WO 2011/088201, WO2011/140425, WO 2015/151001, WO 2013/122897 and Bioorganic & Medicinal Chemistry Letters (2011), 21, 3676-681 disclose compounds related to voltage-gated sodium channel (VGSC) modulators for the treatment of various diseases mediated by VGSC modulation.